Regrowth patterns, defences and allocation of stored energy reserves in Acacia seedlings following herbivory and fire.

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Abstract

Disturbances, such as herbivory and fire are commonplace in savanna ecosystems. The effects of
herbivory and fire on growth and defences of adult trees is a much studied filed in plant ecology.
However, there are comparatively few studies that have investigated the effects of herbivory and
fire on seedling growth, defences and establishment. African Acacia trees are common and
widespread, and are suggested to be keystone species in savanna ecosystems. They have been
shown to significantly positively increase soil characteristics, such as soil moisture and
infiltration, while also increasing spatial heterogeneity of savannas. These trees are
distinguishable by their array of physical and chemical defences. Physical defences are either in
the form of spines (physiologically costly to produce, thus considered inducible) or prickles
(physiologically cheap to produce, thus considered constitutive), while condensed tannins are the
most common form of chemical defences. Adult Acacia trees have been shown on several
occasions to be highly resilient to disturbance events, primarily due to their large size. However,
the effects of herbivory and fire on Acacia seedlings have been little studied despite their
apparent importance for our understanding of African savanna ecosystem functioning.
In two separate experiments, this thesis aimed to investigate the individual and combined
effects of simulated herbivory and fire on the regrowth, defences, and total non-structural
carbohydrates (TNC) of the seedlings of several Acacia species, while also testing the Resource
Availability Hypothesis (RAH) (14 Acacia species) and the Expanded Growth-Differentiation
Balance Hypothesis (GDBH) (three Acacia species). The RAH (Coley, Bryant & Chapin 1985)
and the GDBH (Loomis 1932) both assume that arid-adapted (resource-poor) species are slow
growing, with low tissue turnover rates. Contrastingly, humid-adapted (resource-rich) species
have fast growth rates and thus a high tissue turnover rate. Therefore, arid-adapted species are
predicted to invest more carbohydrate reserves in defence after a disturbance event, in order to
defend new photosynthetic material. Conversely, humid-adapted (resource-rich) species are
predicted to invest carbohydrate reserves into increased growth after a disturbance event, in order
to compensate for tissue loss.
The first greenhouse experiment found that, in accordance with the RAH, humid-adapted
species displayed elevated growth rates compared to arid-adapted species. Overall, defences
significantly increased after herbivory, but significantly decreased after fire. Herbivory was also shown to significantly reduce TNC stores in roots more than fire. We found that arid-adapted
species did indeed invest more carbohydrate stores (TNC) into defence, and displaying an
increase in spine and prickle abundance, spine and prickle length, and condensed tannin
concentration. Humid-adapted species displayed an increase in growth rather than in defence,
with the majority of species only increasing one defensive trait. Humid-adapted species also
displayed significant trade-offs (negative correlations) between new stem growth and defence
traits, while arid-adapted species overall did not display any significant trade-offs between stem
growth and increased defences. The majority of arid-adapted species also displayed an allometric
effect for spine abundance and length, with an increase in one trait led to an increase in the other.
Prickles were found to be inducible, despite the assumption that due to low physiological cost,
they are no inducible. Herbivory and fire were shown to not be substitutable in their effects on
Acacia seedlings in a controlled experimental setting.
The second greenhouse experiment tested the Expanded Growth-Differentiation Balance
Hypothesis (GDBH) in the seedlings of A. erioloba, A. karroo, and A. nilotica using five levels
of nutrient availability. Overall, spine abundance and spine length displayed a unimodal trend in
all three species, with spine abundance and spine length being greatest at a nutrient availability
of 800 mg/ ℓ. Spine abundance, spine length and condensed tannins increased significantly after
herbivory, but were shown to significantly decrease after fire. We found that with an increase in
nutrient availability, the growth of stems and roots, along with the production of TNC in roots
and stems of all species significantly increased. Our data provide mixed support for the
assumptions and predictions of the expanded GDBH. The regrowth of stems and physical
defences were consistent with the GDBH. Chemical defences (i.e. condensed tannins) were
however, inconsistent with the predictions of the GDBH.
We have shown that Acacia seedlings are highly resilient to disturbance events, while
the growth responses of Acacia seedlings are underpinned by TNC stores in roots. Simulated
herbivory and fire are often substituted for one another in controlled experiments. However, we
have shown that the effects of herbivory and fire have significantly different effects on regrowth
patterns of Acacia seedlings.

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